Many chemical and petrochemical reactions are performed in the presence of catalysts comprising transition metals, such as platinum or cobalt, which are very expensive. Attempts have thus been made to propose particles comprising one of these transition metals mixed with a less expensive metal.
As far as cobalt is concerned, it has been suggested in EP 0 261 870 to use in the conversion of syngas to hydrocarbons (in the so-called Fischer-Tropsch process), after reductive activation, a catalyst made from core-shell particles having a core of zinc oxide and a shell comprising cobalt oxide. These particles may be produced either by co-precipitation of insoluble thermally decomposable compounds of zinc and cobalt, or by precipitation of an insoluble thermally decomposable compound of cobalt in the presence of zinc oxide. Typically, the insoluble thermally decomposable compounds are formed from metal oxides obtained by adding a precipitant such as a base to an aqueous solution of the corresponding metal salts. The drawback of the co-precipitation method is that the size of the particles cannot be properly controlled, which in turn detrimentally affects the conversion rate and selectivity of the catalytic process in which these particles are used.
Moreover, the catalytic activity of the particles obtained according to these two methods need to be improved. Another method to prepare core-shell catalysts has been proposed in U.S. Pat. No. 7,422,995 and is referred to as the layer-by-layer (or “LBL”) method. Again, the catalytic activity of the particles can be improved. Moreover, it involves several steps and the use of a surfactant to anchor the cobalt layer to the chemically inert core coated with a zinc oxide layer, which increases the cost of this method.
Other core-shell materials have been proposed for catalytic applications (J. BAO et al., Angewandte Chemie, International Edition in English, vol. 47, pp. 353-356, 2008; J. M. BADANO et al., Applied Catalysis A., Vol. 390, pp. 166-174, 2010). They include an active phase, generally a transition metal, which is set as the core of the composite particle. This active core is then coated with a protective shell, such as mesoporous silica, titania or carbon nanotubes. Covering the active phase with a protective shell can prevent sintering, while allowing reactants and products to diffuse through the catalyst. However, the system is severely affected by the diffusion limitation across the shell. Examples of such core-shell catalysts have been used in Fischer-Tropsch reactions (R. XIE et al., Catalysis Communications, Vol. 12, pp. 380-383 and pp. 589-592, 2011).
Therefore, there remains the need to provide a cost-effective method for preparing core-shell metal particles intended to be used in the manufacture of a catalyst which comprises a much lower amount of cobalt than known catalysts while having at least the same catalytic activity.
This need has been satisfied by a novel method which leads to specific particles having a core of iron oxide and a shell comprising cobalt. Moreover, to the inventors' knowledge, these particles have never been described before.
Specifically, core-shell particles having a core of iron oxide and a shell comprising cobalt, and other cobalt-doped iron oxide particles, have already been described in various documents such as U.S. Pat. Nos. 6,080,233, 5,512,317, 5,484,628, 5,183,709, 4,276,183, 4,226 909, 4,420,537 and U.S. Pat. No. 3,573,980. These particles are intended to be used as magnetisable particles in magnetic recording tapes which require both a high coercivity and a good orientation ratio of the particles in a binder. These particles have an acicular shape, resulting from their preparation processes. These processes use acicular iron oxides as starting materials, which are treated in a basic aqueous solution so as to form a core of magnetite, berthollide or γ-Fe2O3, having a size of more than 20 nm and most often more than 100 nm. A cobalt salt (and optionally other metallic salts or a silicate) is added to the iron compound either before or after the formation of the core, so as to result in a shell comprising cobalt and optionally iron or chromium (and optionally a silicate). Similar processes have been disclosed by A. E. Berlowitz et al in IEEE Transactions on Magnetics, Vol. 24, No. 6, November 1988, by M. KISHIMOTO et al in IEEE Transactions on Magnetics, Vol.Mag-21, No. 6, November 1985, by H. SESIGUR et al. in Materials Research Bulletin, Vol. 31, No. 12, pp.1581-1586, 1996 and by K. SAKAI et al. in J. Appl. Crystal., Vol. 34, pp. 102-107, 2001.
The above prior art does not suggest that spherical particles, having a core of iron oxide of less than 100 nm, and even less than 20 nm, and a shell comprising cobalt, can be produced. Moreover, these documents do not suggest the simple and inexpensive method of this invention, which may be carried out to produce these particles with a controlled shell thickness.